I just wondered (as I am pondering paying for a seat on BA) what would happen if you paid for a seat and when it came around to the actual flight you were instructed (as opposed to being asked) to move?

Would you lose your money, be offered an equivalent seat (for example maybe an exit row on the other side of plane etc), be given a refund or be given miles as compensation?

I'm just curious if it has ever happened to anyone, or if not, what would happen?

Cheers